This invention relates to a torque control system of an induction motor.
In most cases, DC shunt motors have been used in such field requiring quick response characteristics as position control apparatus and speed control apparatus. Because a quick response control system can be realized since the output torque is proportional to the armature current and since flow of the armature current immediately produces the output torque, and because a control system intended by a designer can readily be realized with a simple linear automatic control theory.
Where induction motors abundantly used as constant speed motors can be used in these control systems there arises many advantages over direct current motors. For example, as no brush is used not only maintenance is easy but also electric spark, electric noise and mechanical fault can be avoided. Further, as no commutator is used problems of commutation do not occur so that large current high speed operation is possible. Moreover, induction motors can be manufactured to be dust proof and explosion proof with small and low cost.
Among the prior art induction motor control systems is included so-called V/f constant control system in which the primary voltage and the primary frequency of the motor are made proportional to the number of rotations. With this control system, however, it is difficult to immediately produce an instructed torque thus failing to realize a quick response control system, and to produce torque according to linear automatic linear control theory.